The present invention relates to a vehicle speaker.
Vehicle speakers of various structures and layouts are available. In one example of a speaker layout, a full range speaker is arranged at the lower part of a door, and a high-frequency speaker (tweeter) is arranged at the base of a front pillar. Japanese Laid-Open Patent Publication No. 2005-277471 describes a speaker using a giant magnetostrictive material to vibrate a floor or road.
In an automobile, a high-frequency sound of, for example, 1 kHz or higher, is apt to being affected by the position of a seat or the height of a vehicle occupant. Thus, in the prior art, a high-frequency sound is heard differently by a vehicle occupant depending on the seat position, the height of the vehicle occupant, or the like. Such difference in how a high-frequency sound is heard is caused by factors (a) and (b), which are listed below.
(a) High-frequency sound has a strong linear propagation property, and a high-frequency sound speaker has a high directivity property.
(b) The vehicle includes obstacles, such as seats, passengers other than the driver, and glass windows, which easily reflect sound. Due to the interference, reflection, and absorption of sound in the passenger compartment, sound may be heard as being loud in one region and soft in another region.
More speakers widen the area reached by high-frequency sound in comparison with when there are fewer speakers. However, due to the properties of a high-frequency speaker, which is mentioned above in factor (a), improvements cannot be made to how high-frequency sound is heard from each seat. In addition, the use of more speakers would cause sound interference (phase interference) that leads to the distortion of sound.
Further, when using speakers of the prior art, a sound image is fixed at a low position. It is preferred that a sound image be fixed at a position located slightly above the dashboard. The position at which a sound image is fixed refers to a position perceived by an occupant as, for example, where an instrument is being played. However, when using speakers of the prior art, it would be difficult to fix the sound image at a high position. In other words, it would be difficult to raise the acoustic field. The difficulty in raising the acoustic field is due to factors (c) and (d).
(c) A full-range speaker, which has a broad directivity, cannot be installed at a high position in the passenger compartment due to the structure of the vehicle.
(d) Due to the properties of a high-frequency speaker, the directivity of sound is high.
To raise the acoustic field, a speaker system may include a plurality of high-frequency sound speakers arranged from near the middle of an A-pillar to near an assist grip for a rear seat. Alternatively, a speaker may use part of a head lining as a diaphragm. However, due to the above-mentioned factor (d) and the factor in that the distance from the speaker to the ears of a vehicle occupant is too short, such structures would not be able to sufficiently raise the acoustic field.